Fossil fuel combustion produces a large quantity of flue gases which is discharged into atmosphere, wherein besides sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides and a small quantity of harmful organic substances, a large quantity of dusts is contained. There are tiny hydrophilic and non-hydrophilic particles (mainly calcium salt particles, aluminum salt particles, magnesium salt particles, titanium salt particles, iron salt particles, lead salt particles, zinc salt particles, cobalt salt particles, rare earth element particles, radioactive element particles and particles of other harmful elements, as well as mineral particles such as silica particles, mullite particles, silicate particles, phosphate particles and the like) in these dusts. These particles are discharged together with the flue gases into atmosphere. At the same time, sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides, harmful organic substances, bacteria, and the like are readily adsorbed on the surface of these particles, thus the content of atmospheric suspended particles (which are generally referred to as PM100, PM10, PM2.5, etc.) is increased significantly, resulting in the phenomena of haze and atmospheric photochemical reactions, and causing serious environmental pollution. Therefore, research on flue gas desulfurization and denitration is quite urgent and also challenging.
At present, desulfurization process and denitration process of flue gases are independent of each other. Typically, denitration is followed by desulfurization.
For the existing desulfurization process, there are mainly desulfurization process of hearth calcium-spraying combustion method as well as dry desulfurization process and wet desulfurization process for flue gases, industrial raw material gases and other waste gases containing sulfur, and so on.
In the dry desulfurization process, a flue gas passes through a dry desulfurizer before being vented, and is vented after being desulfurized. The dry desulfurizer is filled with a solid particulate dry desulfurization agent, which is usually iron oxide, zinc oxide, manganese oxide, cobalt oxide, chromium oxide, molybdenum oxide, and the like. Such dry desulfurization agent absorbs the sulfur dioxide in the flue gas and then generates corresponding sulfite. When such oxide loses its ability to absorb sulfur dioxide, it has to be replaced with a new dry desulfurization agent. The consumption of dry desulfurization agent is extremely large, and it is expensive, while a large quantity of waste residues is produced at the same time, which need to be piled up and landfilled, resulting in the phenomena of serious pollution transfer and secondary pollution.
The desulfurization agents used in the wet desulfurization process are mainly calcium carbonate, magnesium carbonate, calcium oxide, magnesium oxide, ammonia, sodium (or potassium) sulfite, organic acid-organic acid salt buffer solution, and the like.
In the desulfurization process thereof, calcium carbonate, magnesium carbonate, calcium oxide, magnesium oxide, and the like are ground into fine powders with a mesh number greater than 325 mesh, and then formulated into a slurry containing 10%-15% calcium carbonate powder. In an absorption tower, the slurry contacts with the flue gas, and sulfur dioxide in the gas reacts with calcium carbonate or magnesium carbonate or calcium oxide or magnesium oxide in the slurry to produce calcium sulfite or magnesium sulfite.
In the air forced oxidation layer of the desulfurization tower, the calcium sulfite slurry is oxidized to calcium sulfate, thus the calcium sulfite slurry is converted to a calcium sulfate slurry. The calcium sulfate slurry flows out of the absorption tower and enters into a separator to separate calcium sulfate from the slurry. Water separated out is returned for recycle use to prepare calcium carbonate slurry. Calcium sulfate separated out is either landfilled as a waste residue or is used for roadbed repairing, or it can be purified and used for making plasterboards. However, this process generates a lot of solids of calcium sulfate, which also contain solid wastes such as some calcium sulfite, unreacted calcium carbonate, and the like. With large limestone consumption, huge devices of crushing and milling, and high power consumption, the investment is great. Moreover, solid precipitates, which tend to clog the devices, are produced during the absorption process. Sewage discharge is large, and secondary pollution is serious.
As for the magnesium sulfite slurry, it is subjected to precipitation and enters into a separator to separate magnesium sulfite from the slurry. Water separated out is returned for recycle use to prepare magnesium oxide slurry. Magnesium sulfite separated out is calcined at a temperature above 1235K to produce magnesium oxide and gaseous sulfur dioxide. Magnesium oxide produced is used repeatedly, and sulfur dioxide can be recycled. However, the magnesium sulfite solids produced in the process tend to clog the pipelines and devices, and also, magnesium sulfite does not decompose until it is calcined at a temperature above 1235K, thus energy consumption is high, investment is large, and secondary pollution is serious.
Meanwhile, tiny hydrophilic and non-hydrophilic particles (mainly calcium salt particles, aluminum salt particles, magnesium salt particles, titanium salt particles, iron salt particles, lead salt particles, zinc salt particles, cobalt salt particles, rare earth element particles, radioactive element particles and particles of other harmful elements, as well as mineral particles such as silica particles, mullite particles, silicate particles, phosphate particles and the like) contained in the slurry of calcium carbonate, magnesium carbonate, calcium oxide, magnesium oxide, and the like are carried out by the flue gas, and discharged into atmosphere, while sulfur dioxide, sulfur trioxide, hydrogen chloride, hydrogen fluoride, nitrogen oxides, harmful organic substances, bacteria, and the like are readily adsorbed on the surface of these particles, thus the content of atmospheric suspended particles (which are generally referred to as PM100, PM10, PM2.5, etc.) is increased significantly, resulting in the phenomena of haze and atmospheric photochemical reactions, and causing serious environmental pollution.
In ammonia desulfurization process, ammonia is used as the desulfurization agent, and an aqueous ammonia of a certain concentration is used as an absorbent to absorb the sulfur dioxide in the flue gas, ammonium bisulfite is generated from the reaction, and is converted to ammonium bisulfate by forced oxidation. Ammonium bisulfate is finally basified to ammonium sulfate by ammonia, and ammonium sulfate is separated as a fertilizer by-product. However, ammonia has high toxicity, very strong volatility and low boiling point, thus the phenomenon of ammonia entrained in the flue gas is serious, resulting in heavy loss of ammonia and secondary atmospheric pollution caused by ammonia. At the same time, the consumption of ammonia is very large, and ammonia is converted to low-valued ammonium sulfate, thus the cost of desulfurization is high, and crystallization is likely to occur, clogging devices and pipelines. Moreover, ammonia is highly corrosive, thus the device corrosion is serious, and ammonia production process is a process of high energy consumption and high pollution, excessive consumption of ammonia is actually a process of indirectly worsening environmental pollution and increasing energy consumption, therefore the use of ammonia should be minimized.
In seawater desulfurization process, a large quantity of salts of calcium, magnesium, aluminum, sodium, potassium and other ions contained in the seawater, especially carbonates, is used as desulfurization agents, and these desulfurization agents react with sulfur dioxide to produce sulfite. Seawater with absorbed sulfur dioxide is subjected to sun exposure and oxidation, wherein the sulfite is oxidized to sulfate, and then discharged directly into the sea. This technology and process can only be applied at seaside, because the seawater consumption is particularly large. As a large quantity of sulfates is discharged into the sea, seawater environment is more or less damaged.
In sodium (or potassium) sulfite process, sodium (or potassium) sulfite is used as the desulfurization agent, and in an absorption tower, its aqueous solution contacts with the flue gas. Sulfur dioxide in the gas reacts with sodium (or potassium) sulfite in the solution to produce sodium (or potassium) bisulfite, thus the sodium (or potassium) sulfite solution is converted to a sodium (or potassium) bisulfite solution. The sodium (or potassium) bisulfite solution is regenerated by heating, and the sodium (or potassium) bisulfite solution is converted to aqueous solution of sodium (or potassium) sulfite, and sulfur dioxide is released at the same time. The aqueous solution of sodium (or potassium) sulfite is recycled for use, and sulfur dioxide gas can be recovered as a by-product. The process is better in terms of conception, however, practical tests show that the regeneration rate by heating of sodium (or potassium) bisulfite solution is very low, only a very small part of sodium (or potassium) bisulfite can be converted to sodium (or potassium) sulfite, most of sodium (or potassium) bisulfite cannot be decomposed, and steam consumption is large. Therefore, industrialization of the process has always been unachievable.
Wellman-Lord desulfurization process is actually an improved sodium sulfite circulation process, but with a multi-effect evaporation procedure added thereto, for the evaporation of desulfurization solution to crystallize sodium sulfite. There are large-scale application examples of this process in the United States, Germany, France and Japan, but its steam consumption is large, and energy consumption is high.
In the organic acid-organic acid salt buffer solution desulfurization process, a buffer solution comprised of organic acid and organic acid salt is used as a desulfurization solution, and in an absorption tower, it contacts with the flue gas, and sulfur dioxide in the gas reacts with the organic acid salt in the solution to produce bisulfite and organic acid. The solution is regenerated by heating, the sulfite in the solution is converted to organic acid salt, and the solution is still converted to the organic acid-organic acid salt buffer solution for repeated use. At the same time, sulfur dioxide is released, and the sulfur dioxide gas can be recovered as a by-product. The process is better in terms of conception, however, practical tests show that the regeneration rate of sulfite in the organic acid-organic acid salt buffer solution is very low during steam heating, only a very small part of sulfite can be converted to organic acid salt, most parts of sulfite cannot be decomposed, and steam consumption is very large. Therefore, the industrialization of the process has always been unachievable. On the basis of this process, a suggestion is that calcium oxide (or calcium hydroxide) is added to the organic acid-organic acid salt buffer solution, such that unregenerated sodium sulfite is converted to calcium sulfite, which precipitates and is separated, thus the solution is thoroughly regenerated. The regenerated organic acid-organic acid salt buffer solution is recycled for use, but the actual desulfurization agent in the process is still calcium oxide (or calcium hydroxide). As the solution contains some calcium ions, precipitation may occur during desulfurization, clogging pipelines and devices.
At present, the denitration process mainly used in the actual production is selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR).
In selective catalytic reduction (SCR), a catalytic bed or system is utilized to process a flue gas stream, wherein ammonia or urea is injected into the flue gas and mixed, then the mixture is passed through a catalyst layer, and NOx is selectively converted (reduced) to N2 and H2O. SCR method is currently the most proven denitration technology with the highest denitration efficiency. The first demonstration project of the SCR system was established in Shimoneski power plant in Japan in 1975, afterwards the SCR technology was widely applied in Japan. In Europe, there have been successful application experiences from more than 120 large-scale devices, and the NOX removal rate can reach 80%-90%. So far, there are approximately 170 sets of devices in Japan, power plants with a capacity of close to 100 GW have installed such apparatus, and US government also uses the SCR technology as the main technology for the main power plants to control NOX. It is reported that the SCR method has currently become a relatively proven mainstream technology for denitration in power plants at home and abroad. The principle of flue gas denitration by the SCR method is as follows: under the catalysis of catalyst with TiO2 and V2O5 as the main components and at a temperature of 280-400° C., or under the catalysis of catalyst with TiO2, V2O5 and MnO as the main components and at a temperature of higher than 180° C., ammonia is sprayed into the flue gas, and NO and NO2 are reduced to N2 and H2O, to achieve the purpose of denitration.
The SNCR denitration technology is a selective non-catalytic reduction technology without the use of catalysts, wherein at a temperature in the range of 850-1100° C., an amino-containing reducing agent (such as aqueous ammonia, urea solution, etc.) is sprayed into a furnace, and NO and NO2 in the flue gas are reduced to N2 and H2O, thus the purpose of denitration is achieved. However, NOX removal rate of the industrial SNCR system is only 30-70%.
Both in SCR and SNCR denitration processes, ammonia consumption is relatively large. As the flue gas contains about 4%-9% O2, ammonia gas or amino-containing urea will react with O2 to produce NOX, ammonia is thus consumed, meanwhile ammonia reacts incompletely, some ammonia is discharged into atmosphere together with the flue gas, and the loss of ammonia increases, resulting in the phenomenon of secondary pollution. A large quantity of fossil fuels is consumed during ammonia production, and a large quantity of waste gases, waste residues and waste water is produced, which is a severe process of environmental pollution, thus the use of ammonia should be avoided as far as possible.
There are also some drawbacks in the existing methods for removing NO from flue gases by SCR and SNCR. For the NO removal methods with ammonia as the reducing agent, ammonia, urea or aqueous urea solution is generally used as the source of the reducing agent. Excessive injection of ammonia or urea will lead to the so-called ammonia penetration, and the discharged ammonia is even more harmful than the discharged NOX. The oxidation of excessive ammonia may lead to the formation of NOX, and the transportation and storage of ammonia reducing agent have high requirements for safety and environmental protection. In addition, the catalyst used in the process of denitration will suffer from impingement and abrasion by high-concentration smoke and contamination by impurities in fly ashes. Excessively high temperature of flue gas will lead to catalyst sintering and deactivation, and the presence of SO2 will lead to a rapid decline in catalyst activity.
Both in SCR and SNCR denitration processes, ammonia consumption is relatively large. As the flue gas contains about 4%-9% O2, ammonia gas will react with O2 to produce NOX, ammonia is thus consumed, meanwhile ammonia reacts incompletely, some ammonia is discharged into atmosphere together with the flue gas, and the loss of ammonia increases, resulting in the phenomenon of secondary pollution. A large quantity of fossil fuels is consumed during ammonia production, and a large quantity of waste gases, waste residues and waste water is produced, which is a severe process of serious environmental pollution, thus the use of ammonia should be avoided as far as possible.
Many researchers at home and abroad propose to use ozone for simultaneously oxidizing SO2 and NO in a flue gas to SO3 and NO2, and then lime/limestone, sodium hydroxide, etc. are used for absorption, thus achieving the effect of simultaneous removal of SO2 and NO. However, since ozone-generating device is very expensive, a great investment is required; and ozone production cost is very high, that for the oxidation of 1 mole of SO2 to SO3 or of 1 mole of NO to NO2, the ozone consumption required is 1.5-3 moles, respectively, while for producing 1 kg of ozone, about 10 kWh of electricity and 10-20 kg of pure oxygen are to be consumed, respectively; the energy consumption is large, the expenditure is high, and the investment is great, making the large-scale industrialization of flue gas desulfurization and denitration by ozone unachievable currently.
CN101352645A discloses a denitration process by catalytic oxidation, wherein the catalyst uses TiO2 or ZrO2—TiO2 as the carrier and Co as the active component. NO is oxidized to water-soluble NO2 by the oxygen contained in the flue gas itself, and then an alkaline solution is used for absorption and nitrogen oxides are thus removed.
CN1768902A discloses a boiler flue gas denitration method, wherein ozone O3 is sprayed into a low-temperature section in a temperature range of 110-150° C. of the boiler flue, and nitric oxide NO in the boiler flue gas is oxidized to water-soluble nitrogen oxides of high valences, such as NO2, NO3 or N2O5; the molar ratio of the sprayed ozone O3 to NO in the boiler flue gas is 0.5-1.5, and then the nitrogen oxides in the flue gas are removed by washing with an alkaline aqueous solution. However, in actual use, this technology has relatively low denitration efficiency and very high ozone consumption. To meet emission standards, its operating cost is particularly high, and enterprises cannot afford it, so large-scale industrialization of this technology has always been unachievable.